The present invention relates to the field of radio communications networks. More specifically, the present invention relates to determining neighbor lists for code division multiple access (CDMA) sectors in a radio communications network for use in cellular network planning tools.
As cellular communication has become more popular, cellular service providers have felt increasingly pressured to use the cellular radio frequency spectrum as efficiently as possible. Greater efficiency allows a service provider to carry more calls using a given amount of RF spectrum. Code division multiple access (CDMA) is an emerging form of digital cellular service. CDMA is a spread spectrum technology that assigns a unique code to all speech and data bits from a user and transmits the encoded speech bits over the same broad shared frequency spectrum. A receiver demodulates the encoded speech and data signals out of the radio frequency environment and reassembles the speech/data to its original format.
The advantages of CDMA over conventional analog radio communications networks include increased capacity and more efficient use of the radio frequency spectrum. In addition, CDMA technology provides three features that improve system quality. These features include xe2x80x9csoft handoffxe2x80x9d, variable rate vocoding, and multipath signal processing techniques. The soft handoff feature, also known as xe2x80x9cmake before breakxe2x80x9d, ensures that a call is connected to the destination cell or sector before handoff is completed, thus reducing the probability of a dropped call. The variable rate vocoding feature facilitates speech/data bits to be transmitted at only the rates necessary to ensure high quality signals and only when speech/data is present. The bit rate reduces during silent periods in the conversation, thus conserving the battery power of the mobile station. The multipath signal processing techniques combine power for increased signal integrity and lower required signal levels.
In a CDMA radio communications network, each CDMA base station has associated therewith a pilot channel carried over a coded channel, or code space, of a forward CDMA channel. A pilot channel is an unmodulated, direct-sequence signal broadcast continuously by each CDMA base station. The pilot channel allows a mobile station to acquire the timing of the forward CDMA channel, provides a phase reference for coherent demodulation, and provides a mechanism for making signal strength comparisons between base stations for determining when to request a hand off.
An active mobile station in a CDMA network maintains four sets of pilot channels, an active set, a candidate set, a neighbor set, and the remaining set. The active set includes the pilot channels that the mobile station is currently using for demodulation. The candidate set includes the pilot channels that are not currently in the active set but are received by the mobile station with sufficient pilot signal strength. The neighbor set includes the pilot channels that are not in the active set or the candidate set, but which may become eligible for handoff. The remaining set is the set of all possible assigned pilot channels in the CDMA network, on the same carrier frequency, excluding the pilot channels in the active, candidate, and neighbor sets.
Handoff is a process by which a mobile station conversation is passed from one cell or sector to another cell or sector. Currently, base station equipment allows handoff only with those pilot channels that are present in a neighbor list associated with a CDMA cell or sector of a CDMA cell, and maintained by the switch for the CDMA cell or sector. Thus, a neighbor set for an active mobile station is formed from the neighbor list for the CDMA cell or sector in which the mobile station is currently xe2x80x9cresidingxe2x80x9d.
In a CDMA radio communications network, several types of handoff are possible. One such type of handoff is a single handoff. Single handoff occurs when the mobile station is connected to only one CDMA cell or sector. Another type of handoff is a two-way handoff. A two-way handoff could occur in one of two ways, a xe2x80x9csofterxe2x80x9d handoff or a xe2x80x9csoftxe2x80x9d handoff. The two-way softer handoff occurs when the mobile station is connected to two sectors of the same co-located cell site. The two-way soft handoff occurs when the mobile station is connected to a sector from each of two different cell sites.
Yet another type of handoff is a three-way handoff. A three-way handoff could occur in one of three ways, a soft/softer handoff, a three-way soft handoff, and a three-way softer handoff. The soft/softer handoff occurs when the mobile station is connected to three sectors of two cell sites, that is, two sectors of one cell site and one sector of the second cell site. A three-way soft handoff occurs when the mobile station is connected to three sectors of three different cell sites, and a three-way softer handoff occurs when the mobile station is connected to three sectors of the same cell site.
In a CDMA network, the single handoff case requires the highest forward link requirement in terms of received signal to noise ratio, Eb/No. When the mobile station is in a two-way or three-way handoff more than one signal is used in demodulation. Accordingly, the multi-user interference is less and the link requirement for the same quality of service is reduced. The amount of reduction depends on the relative signal strengths of the handoff pilot channels with respect to the current serving pilot channel from the active set and various receiver implementation losses.
In CDMA, a pilot channel from another sector will be included in the demodulation process only if it is included in the neighbor list of the current serving sector. If a pilot channel of sufficient strength from another sector is not included in the neighbor list, then it adds to the interference. The added interference can result in dropped calls, higher frame error rates (FER), and a poorer quality of service. Since pilot channels of sufficient strength that are not in the neighbor list can lead to poor quality of service, it is highly desirable to accurately determine and define the appropriate neighbor list for each CDMA cell or CDMA sector in a CDMA network.
When CDMA cell sites are co-located with analog cell sites, one approach to determining the CDMA neighbor list is to utilize the neighbor lists generated for the analog radio communications network. An analog neighbor list is based on the signal strength of the signals from every sector and the interference at every location. The analog neighbor list is a ranking in terms of the signal strength threshold, such as, carrier-to-interference (C/I) ratio.
An analog system, such as the advanced mobile phone system (AMPS), utilizes different radiofrequencies in adjacent cells or sectors. As such, an analog system employs a xe2x80x9cbreak and makexe2x80x9d handoff technique requiring the connection to be broken in the original cell before it is made in the successor cell. Thus, handoff in an analog network desirably utilizes a channel from the analog neighbor list having the greatest C/I ratio. Other channels from the analog neighbor list, for example, the channel in the analog neighbor list having the second greatest C/I ratio, may be considered an interfering signal in an analog system.
Unfortunately, such an analog neighbor list ranked in terms of C/I ratio is undesirable for use as a neighbor list in a CDMA network. An analog neighbor list is undesirable because it does not take into account the various nuances of CDMA, such as the receiver type and its ability to demodulate up to three signals simultaneously. A channel that may be considered to be interfering in an analog system may help the primary pilot channel into a better situation (i.e. a two-way or three-way handoff) in a CDMA system.
Thus, what is needed is a method and system for determining CDMA neighbor lists that advantageously exploit the soft handoff and multipath signal processing features of the CDMA technology.
Accordingly, it is an advantage of the present invention that a method and system for determining a neighbor list for a code division multiple access (CDMA) sector in a radio communication network are provided.
It is another advantage of the present invention that the method and system take into account the underlying CDMA network performance and substantially each possible mobile station location.
Yet another advantage of the present invention is that candidate handoff channels in the neighbor list of the CDMA sector are ranked such that candidate handoff channels having a higher rank have a greater likelihood of being selected for inclusion in the neighbor set of a mobile station.
The above and other advantages of the present invention are carried out in one form by a computer-based method of determining a neighbor list for a code division multiple access (CDMA) sector in a radio communications network, the network further including neighbor CDMA sectors proximate the CDMA sector. The method calls for detecting, at locations in the CDMA sector, neighbor broadcast channels transmitted from the neighbor CDMA sectors. For each of the locations, a first broadcast channel is distinguished from the neighbor broadcast channels detected at the location. For each location, a signal quality threshold window is established in response to a signal quality of the first broadcast channel. Second broadcast channels are identified from the neighbor broadcast channels, each of the second broadcast channels is identified when a signal quality of each second broadcast channel is within the signal quality threshold window. The first and second broadcast channels are included as candidate handoff channels in the neighbor list for the CDMA sector.
The above and other advantages of the present invention are carried out in another form by a computing system for determining neighbor lists for each of a plurality of code division multiple access (CDMA) sectors in a radio communication network. The neighbor lists are utilized to form neighbor sets of candidate handoff channels for mobile stations residing in associated ones of the CDMA sectors. The computing system includes an input element for receiving a pilot channel detection database for the radio communications network. The database includes a plurality of locations for each of the CDMA sectors and pilot channels transmitted from the CDMA sectors and detected at corresponding ones of the locations. A processor, in communication with the input element, creates the neighbor lists of the candidate handoff channels from the pilot channels in the pilot channel detection database, one each of the neighbor lists corresponding to one each of the CDMA sectors. A memory element, in communication with the processor, stores the neighbor lists for the CDMA sectors, the candidate handoff channels in each of the neighbor lists being arranged in a ranked order. The ranked order establishes a likelihood for the neighbor pilot channels to be selected for inclusion in the neighbor sets.